This invention relates to propulsion apparatus for boats and more particularly to bladed propellers which revolve under water to exert thrust on the hull of a boat.
Revolving propellers generate a thrust force on the hull of a boat by forcing a water flow outward from the back of the propeller. The water flow produced by a bladed propeller of conventional design also moves radially relative to the rotary axis of the propeller which effect is most pronounced in the tip region of the blades. The radially directed component or slip of the water flow does not produce forward thrust. This reduces the efficiency of the propeller and unproductively consumes a portion of the power produced by the engine which drives the propeller. The radial slip flow may also reduce water pressure at the back of the propeller thereby further reducing propeller efficiency.
Non-slip propellers have heretofore been designed for the purpose of redirecting radial water flow at the backs of the propeller blades in a manner which causes it to flow in a more horizontal direction. The horizontal flow adds to the thrust produced by the propeller and thereby increases efficiency. Non-slip propellers of this kind have a flange or lip at the outer end of each blade which extends outward along the tip of the blade in position to intercept the radial flow and to redirect it into a horizontal flow. The trailing edge of the flange is typically closer to the rotary axis of the propeller than the leading edge of the flange. This increases water pressure at the backs of the blades and thereby further increases efficiency.
The high efficiency of the non-slip propeller enables use of smaller diameter propellers in instances where that is desirable such as in shallow draft boats. Pre-existing propellers of traditional design can be reduced in size without loss of efficiency by machining them to have a smaller diameter and then adding the slip redirecting flanges to the tips of the blades. Non-slip propellers can also reduce or virtually eliminate noise problems which are sometimes caused by propellers throwing water against the hulls of boats.
Prior non-slip propellers of the above discussed type have not fully realized the gains in efficiency which are potentially possible. In some designs the non-slip flange has an outer surface, facing away from the rotary axis of the propeller, that is curved or angled to extend closer to the rotary axis at its trailing end than at its leading end. Cavitation can cause a water pressure reduction along the outer surface of the flange that increases resistance to rotation of the propeller. Overcoming this increased resistance consumes power produced by the engine which drives the propeller. In other prior designs the non-slip flanges have centers of curvature which are at the rotary axis of the propeller so that the leading ends and trailing ends of the flanges are equidistant from the rotary axis. This flange configuration is not subject to the cavitation effect discussed above but it does not increase water pressure at the back of the propeller in a manner which optimizes efficiency.
The present invention is directed to overcoming one or more of the problems discussed above.
In one aspect the present invention provides a boat propeller having a revolvable hub centered on an axis of rotation and having a plurality of blades extending outward from the hub at equiangular intervals therearound. The blades are pitched to create a thrust producing flow of water through the propeller. Each blade has a curved flange extending along a tip region of the blade and extending outward from the tip region of the blade in position to deflect water which travels towards the tip region outward into the thrust producing water flow. Each flange has an arcuate outer surface which faces away from the axis of rotation and which has a center of curvature situated at the axis of rotation. Each flange has an arcuate inner surface which faces the axis of rotation and which has a center of curvature that is offset from the axis of rotation in a direction which causes the distance between a trailing end of the flange and the axis of rotation to be smaller than the distance between a leading end of the flange and the axis of rotation.
In another aspect the invention provides a boat propeller having an annular revolvable hub centered on an axis of rotation of the propeller. A plurality of blades extend radially from the hub, the blades being situated at equiangular intervals around the axis of rotation and being pitched to create a thrust producing flow of water through said propeller as the propeller turns. Each blade has a curved flange portion extending along the tip of the blade at the outmost region of the blade and extending outward from the tip of the blade in position to deflect slip flow water which travels towards the tip outward into the thrust producing flow of water. Each of the flange portions has an arcuate outer surface which faces away from the hub and which has a center of curvature situated at the axis of rotation. Each flange portion has an arcuate inner surface which faces the hub and which has an offset center of curvature that is offset from the axis of rotation in a direction which causes successive regions of the arcuate inner surface of the flange portion to be progressively closer to the hub.
The invention provides a highly efficient boat propeller of the type which has non-slip flanges at the tips of the propeller blades to convert non-productive radially directed water flow into a more horizontal flow which creates thrust. The outer surface of each flange which faces away from the direction of movement of the flange has a center of curvature which is at the rotary axis of the propeller. This avoids power wasting cavitation effects in the water adjacent to that surface. The inner surface of each flange which faces towards the direction of travel of the flange has a center of curvature which is offset from the rotary axis of the propeller to cause the trailing end of the inner surface to be closer to the center of the propeller than the leading end of the flange. Consequently, the flanges exert pressure against adjacent water which pressure is exerted in the direction of the hub of the propeller. This increases water pressure at the aft face of the hub region of the propeller and thereby inhibits cavitation at that region.
The invention, together with further aspects and advantages thereof, may be further understood by reference to the following description of the preferred embodiments and by reference to the accompanying drawings.